The present invention relates to the manufacture of synthetic polymers and, more particularly, to an improved process for the suspension polymerization of polyvinyl chloride (PVC) and the improved PVC resin produced thereby.
In the suspension polymerization of PVC, vinyl chloride monomer (VCM), water, initiator, buffer, suspending agent and other additives are charged to a polymerization reactor under pressure, and the reaction is allowed to continue over a period of hours until the desired degree of polymerization is achieved. In commercial practice, it is not unusual to use more than one initiator, and/or buffer, and/or suspending agent, and, of course, comonomers and other additives are frequently added to the reactor before initiation of the polymerization reaction, or after partial or even complete polymerization of the vinyl chloride.
The initiator, the suspending agent, the buffers, and other processing aids added to the reactor, as well as the temperature at which the reaction is carried out, can all affect not only the reaction itself but also the product. Thus, the choice of suspending agent can affect not only the facility with which the polymerization reaction proceeds but also many characteristics of the PVC resin produced. The molecular weight, particle size distribution, porosity, bulk density and a wide variety of other properties may thus be affected by the choice of processing conditions and processing additives employed with reaction. The degree to which a given physical property may be truly "controlled" (as opposed to merely affected or altered) by variations in process conditions and/or process additives varies widely, though many are well-known and almost universal in the art, e.g., adjusting reactor temperatures to control molecular weight.
In the past, it was well-known to employ a wide variety of suspending agent, including cellulose ether products such as methyl cellulose and hydroxypropyl methylcellulose, gelatin, inorganic salts, clays and the like and certain highly hydrolyzed polyvinyl alcohols (PVA) to mention but a few. Various combinations of these and other materials have all been employed from time to time in commercial manufacture of PVC suspension resins.
Any attempt to produce end products from pure PVC resin, by itself, would result in an extremely rigid, inflexible, probably useless product, which would have already suffered heat degradation in its formation, and which would be subject to further light degradation if it is kept for any period of time. In order to produce useful products from PVC, it is necessary for the PVC resin manufacturer, end user, or an intermediate "compounder" to admix light and heat stabilizers, in most cases at least some plasticizer, and other optional additives with the PVC resin, to produce a "PVC compound."
For this reason, some of the most important physical properties of PVC resins are not those most readily apparent in the PVC resin, per se, but the properties which affect the further processing of that resin to produce PVC compounds and/or the physical properties of such PVC compound. PVC compounds produced by the resin manufacturer or an intermediate compounder are usually sold either in the form of extruded pellets, or as free-flowing powders, and this form also dictates whether particular properties may be essential, or merely desirable, in the PVC resin to be employed.
In manufacturing free-flowing powder PVC compounds, it is essential to produce a PVC resin which will quickly absorb the liquid additives with which it will be admixed to produce the compound. This is particularly true as to the plasticizer, which usually forms the bulk of the liquid additives and which is normally an oily liquid, such as, for example, dioctylphthalate. The time required for the PVC resin to absorb all of the liquid additives is a function of the porosity of the resin particles and is clearly an extremely important property of a PVC resin intended for such an end use. There are many techniques for determining porosity, some of which are absolute measures, while others are relative. For example, an absolute measure of the interpore volume (IPV) can be obtained by mercury penetration techniques using a porosemeter, as described by ORR, "Applications of Mercury Penetration to Materials Analysis," Powder Technology, 3 (1969/70) 117-123. While there are also many arbitrary procedures to provide a relative measure of this property, the most commonly employed procedure is probably the test for the so-called Brabender Dry-up Time (BDUT).
The BDUT test measures the plasticizer absorption rate of a resin. The absorption rate is relative since it depends on the molecular size of the plasticizer, the mixing temperature, and resin characteristics such as pore volume, pore diameter, and particle size. Plasticizer absorption rate is important in the manufacture of flexible compounds and is critical in dry-blend operations such as fluid bed coating. Therefore, while BDUT is only a relative measure of IPV, it is a direct measure of the ultimate desired property, the facility with which the resin will absorb the liquid additives.
In the BDUT test, the mixing bowl is maintained at a constant temperature. A filled resin mix is added to the mixing bowl and allowed to come to equilibrium before adding the plasticizer-stabilizer mix. Upon addition of the plasticizer, the recorded torque increases sharply. The torque stays at a high level until the excess liquid is absorbed. At this point, the torque begins to decrease and continues to decrease until all of the liquid has been absorbed. The dry-up point is taken as the point on the curve where the torque begins to "line-out"--indicate substantially constant torque.
The test is conducted in a Brabender Plastograph fitting with a 650 ml Sigma Blade Mixing Head. A liquid master batch is prepared by premixing 1905 grams of diisodecyl phthalate, 272 grams of epoxy plasticizer, 136 grams of a liquid barium cadmium stabilizer, and 22.7 grams of a chelator-type stabilizer such as, for example, Mark PL. The ingredients are mixed and maintained at room temperature. The reservoir is preheated to a constant temperature of 89.degree. C..+-.0.5.degree. C., and the Plastograph dial recorder is zeroed with the apparatus running at 63 rpm. Two hundred grams of dry resin (less than 0.3 percent volatiles) and 85 grams of calcium carbonate are blended, charged to the mixing bowl, and stirred until the blend reaches a temperature of 84.degree. C. (about 5 to 10 minutes) at which point 102.+-.1 gram of the liquid master batch are added to the mixer, and agitation is continued until all of the liquid is absorbed. If all of the liquid is not absorbed within 10 minutes, the sample is designated "no dry-up." It should be noted that in any drying of the resin which may be required to meet the 0.3 percent volatiles limit, overdrying should be avoided since this could alter the porosity of the resin. It should also be noted that the standard formulation described above is arbitrarily employed in order to assure validity of the comparative results.
Polyvinyl alcohol suspending agents are usually prepared by hydrolysis (or saponification) of polyvinyl acetate resulting in the replacement of many of the acetate groups by hydroxyl groups, each such replacement having the net result of converting a vinyl acetate monomer unit to a vinyl alcohol unit. The resulting products are normally characterized in terms of (1) level of hydrolysis (or saponification) and (2) standard solution viscosities representative of average molecular weight. The level or degree of hydrolysis is in reality the average mole percent of vinyl alcohol monomer units in the polymer, a figure normally obtained from the "percent residual acetate," which is actually the percent, by weight, of residual vinyl acetate monomer units as determined by an analytical acid-base titration of the excess sodium hydroxide remaining after complete saponification of the partly hydrolyzed polymer in question.
A broad spectrum of such polymers is commercially available as can be seen from the following table of exemplary materials listed by manufacturers' trade names.
TABLE 1 ______________________________________ PVA SUSPENDING AGENTS % Residual Viscosity of Vinyl 4%, by wt., % Hydrolysis Acetate Solution in H.sub.2 O "Trade Names" (molar) (by weight) (centipoises) ______________________________________ Alcotex 25/002 26.3 84.6 * Alcotex 35/002 33.8 79.3 * Alcotex 45/002 43.4 73.5 * Polyvic S202 45.0 70.5 * Gohsenol LL-02 45.0-51.0 65.3-70.5 * Rhodoviol APH 45.0-52.0 64.3-70.5 * Alcotex 50/002 49.6 66.5 * Gohsenol KPZ-04 69.0-73.0 42.0-46.8 2-4 MOWIOL LP5-72 71.0-73.4 41.5-44.4 4-6 Gohsenol KP-08 71.0-75.0 39.4-44.4 6-9 Gohsenol KP-06 71.0-75.0 39.4-44.4 5-7 Alcotex 72.5 71.5-73.5 41.4-43.8 5.5-7.5 Rhodoviol 5/270 71.5-73.5 38.3-42.6 8-10 Gelvatol 40-20 72.9-77.0 36.9-42.1 2.4-3.0 Gelvatol 40-10 72.9-77.0 36.9-42.1 1.8-2.4 Alcotex 73/76L 73.0-76.0 38.2-42.0 5.5-7.5 Alcotex 74L 73.5-74.5 40.1-41.4 5.5-7.5 Gohsenol KH-17N 75.5-77.0 36.9-38.8 39-47 Alcotex 78L 76.0-79.0 34.2-38.2 5.5-7.5 Vinol 620(420) 78.0-81.0 31.4-35.5 38 Gohsenol KH-17 78.5-81.5 30.7-34.7 32-38 Gelvatol 20-30 85.5- 88.7 19.9-24.9 4-6 Gohsenol GH-23 86.5-89.0 19.5-23.4 48-56 Vinol 205 87.0-89.0 19.5-22.6 4-6 Vinol 523 87.0-89.0 19.5-22.6 21-25 Vinol 540 87.0-89.0 19.5-22.6 40-50 ______________________________________ *These low hydrolysis level materials are not sufficiently soluble in plain water to permit equivalent viscosity values to be determined.
In recent years those skilled in the art have been paying more attention than ever to the importance of improving the porosity of vinyl chloride resins and, particularly, to the desirability of achieving improved porosity without significant reductions in bulk density or major shifts or upsets in particle size distribution of the resultant resin. However, in order to avoid the reverse relationship between the directions in which porosity and bulk density usually move when changes are effected, rather complicated dispersing agent systems have been resorted to heretofore.
For example, two exemplary prior art patents are U.S. Pat. No. 3,929,753 to Itoh et al. and U.S. Pat. No. 4,143,224 to Klippert et al. At least two different grades of polyvinyl alcohol suspending agent are used in each of these patents and, in addition, Klippert et al. employ still a third suspending agent of an entirely different type while Itoh et al. initiate polymerization in the presence of only the lower molecular weight PVC with the high molecular weight grade being added later during the course of the polymerization.